If one of the NATO members is an aggressor in a war can the attacked nation invade the aggressor's country without declaring war on NATO?

Yes, battles and wars can be fought without declaring one; even before the advent of Atomic Weapons. The Soviet Union (Russia) and Japan fought a very bloody war at Nomonhan (Battle of Khalkhin Gol, May-September 1939) without a declaration of war. It was during that war that Soviet General Zhukhov learned his trade, and defeated the Germans on the Russian Front during WW2. Declarations of war are normally NOT made when: nations feel it could result in "mutual destruction"; or when they do not want to war with each, but need to settle the issue...and feel that they can settle it quickly; or if the nation is fighting itself and does not recognize the other side's government. The American (US) Civil War (1861-1865) was NOT A DECLARED WAR. useruser (question starter): Yes but NATO's motto is attack on one is translated as attack on all. If a single NATO country is the aggressor and the attacked country invades the aggressor as self-defence, how will NATO view this?